The primary objective of the proposed research is to determine whether metallothionein (MT) is a zinc storage protein. The planned approach is to evaluate the relationship of dietary factors affecting MT levels to zinc metabolism. The main topics to be studied are the effects of zinc on half-life of MT species, the effects of cadmium on half-life of the MT species and zinc storage, the effects of copper, mercury and lead on MT synthesis and zinc storage, the effects of sulfur deficiency on MT synthesis and zinc storage, the determinatin of whether zinc accumulates exculsively in MT fraction of liver and kidneys of swine and sheep similar to rats, to purify and characterize a low molecular weight protein from choroid plexus, and to evaluate by a chemical method the tissue content of MT from animals on various dietary treatments. Radioactive cystine (either S35 or C14 labeled) will be injected into rats fed elevated levels of dietary zinc or cadmium and the tissue MT species isolated for radioactivity measurements at predetermined times afterwards for half-life calculation. To evaluate other factors on MT levels, radioactive cystine will be injected into sulfur or selenium deficient rats, or rats fed diets with high levels of mercury, copper or lead, and the amount of radioactivity incorporated into MT determined. In addition to S35 or C14 labeled cystine, an inhibitor of protein synthesis is planned to be used for evaluation of the extent of reutilization of amino acids derived from MT catabolism. The MT species or low molecular weight protein from choroid plexus are proposed to be purified by salt fraction, gel filtration and ion exchange resin chromatography. An internal standard of purified MT and radioactive mercury are planned to be used for the determination of MT content of tissues.